Acrylamide is considered the most commercially important of the acrylic and methacrylic amides. It is useful, for example, for waste water treatment, soil stabilization, papermaking, manufacture of polymers, and as an additive for textiles, paints, and cement. Industrially, acrylamide is manufactured by acrylonitrile hydration.
Early commercial nitrile hydration was mediated by sulfuric acid. But this environmentally unfriendly process required expensive equipment and presented waste disposal problems. Subsequently, the acid mediated process was abandoned in favor of metal catalyzed hydration. Beginning in 1971, a series of patents issued that described various unsupported elemental copper based nitrile hydration catalysts, some of which have since achieved commercial success; i.e., U.S. Pat. Nos. 3,597,481; 3,631,104; 3,342,894; 3,642,643; 3,642,913; 3,696,152; 3,758,578; and 3,767,706. Since then, hundreds of variations and improvements have been disclosed and the topic has been reviewed (E. Otsuka et al. Chem. Econ. Eng. Rev. 7(4), 29, (1979)).
Of the unsupported elemental copper catalysts, Raney copper is one of the most popular for commercial scale acrylonitrile hydration (See, e.g., U.S. Pat. No. 3,767,706 and U.S. Pat. No. 3,985,806) because of its high surface area and activity per unit copper metal relative to other forms of elemental copper. Preparation comprises leaching an alloy of copper and aluminum with a strong base. But an inherent problem with elemental copper is inefficient metal utilization and therefore high loading is required to achieve reasonable reaction rates and conversions. Another significant drawback of elemental copper hydration catalysts is the limited lifetime, that is, over a period of days to weeks of continuous use, a gradual decrease in activity is observed. Yet another disadvantage is low mechanical stability and catalyst fragmentation into increasingly fine particles. This causes variations in catalyst surface area so that, even if other conditions are maintained, the amount of reaction per catalyst unit will vary and, as a consequence, the conversion rate will be erratic and the catalyst lifetime will be reduced.
Some problems with these elemental metal catalysts can be mitigated, in certain applications, by depositing the metal onto a substrate support. As is well known in the art, supported metals generally have higher surface areas--and thus higher activities--per unit metal than their unsupported counterparts. Hence, with supported catalysts, the metal is used much more efficiently and this, of course, is economically advantageous. One reason for the higher surface area is that the metal can be dispersed on the solid substrate support as small crystallites. This favorable trend, however, is undermined because it is difficult to impregnate supports with small metal crystallites (i.e., having a high metal surface area per unit metal) while simultaneously achieving high metal loading.
The art relating to metals and promoters supported on aluminum oxide (Al.sub.2 O.sub.3 or alumina) is mature and the references are numerous. Activated aluminum oxide is available via dehydration of the hydrated form (Al.sub.2 O.sub.(3-x) (OH).sub.2x). Several types of hydrated aluminum oxide are readily available, including Gibbsite and Bayerite (Al(OH).sub.3). Loss of a molecule of water leads to the oxyhydroxides (AlO(OH)), boehmite, pseudo boehmite, and diaspore (for reviews see: Augustine R. L. , Heterogeneous catalysis for the Synthetic Chemist, Marcel Dekker, Inc. New York, N.Y. (1996) pp. 161-163 and The Encyclopedia of Chemical Technology, 2 Kirk-Othomer (4.sup.th ed. at 426)) both of which are incorporated herein by reference.
Significantly, it is known that aluminum oxide's surface area, mechanical stability, and resistance to hydration are highly dependent upon the temperature at which the aluminum oxide is calcined. In general, as hydrous aluminum oxide is calcined, water is driven off, leaving a porous solid structure of activated aluminum oxide. Simple drying of aluminum oxide at temperatures lower than about 500.degree. C. affords aluminum oxide that has relatively low mechanical stability and low water reabsorption resistance. On the other hand, calcination between about 550.degree. C. to 850.degree. C. affords the more mechanically stable and hydration resistant gamma aluminum oxide phase (.gamma.-alumina), which has a surface area of about 150 to 300 m.sup.2 /g. Further heating (875 to 1150.degree. C.) effects further phase shift through delta-alumina (.delta.-alumina), to theta-alumina (.theta.-alumina), and finally to the alpha-alumina (.alpha.-alumina) phase. This phase change--i.e., .gamma.-,.delta.-,.theta.-, to .alpha.-alumina--is accompanied by increased mechanical stability and increased hydration resistance. Resistance to rehydration is very important to a catalyst's stability and lifetime under aqueous phase reaction conditions (e.g., in hydration reactions). High resistance to rehydration correlates with improved structural integrity. But on the downside, the phase change through .gamma.-, .delta.-, .theta.- to .alpha.-alumina is accompanied by surface area reduction (i.e., to about 5 m.sup.2 /g for .alpha.-alumina). Accordingly, for the purposes of the present invention, the phrase: mechanically stable aluminum oxide phase; refers to either .gamma.-,.delta.-,.theta.-, or .alpha.-alumina or any mixture thereof.
In short, .gamma.-,.delta.-,.theta.-, and .alpha.-alumina are favorable supports in view of their good hydration resistance and high mechanical stability. But a disadvantage of .gamma.-,.delta.-,.theta.-, and .alpha.-alumina as catalyst supports is that the derived metal catalysts are expected to have relatively low activity per unit copper metal.
Disclosures of catalyst compositions comprised of elemental copper and copper oxide supported on aluminum oxide and preparation methods are widespread. In general, a copper salt is bonded to the aluminum oxide support and the support-copper salt complex is calcined to convert the copper salt to copper oxide. If desired, the copper oxide crystallites may be converted to elemental copper by chemical reduction.
One popular procedure for preparation of aluminum oxide supported copper oxide based catalysts is the single pore volume impregnation procedure (or PVI, also known as the incipient wetness procedure). One variation of the pore volume impregnation procedure comprises saturating the pores of an aluminum oxide phase with aqueous copper salt solution, drying, then calcining the impregnated aluminum oxide to convert the copper salt to copper oxide. Unfortunately, this single impregnation procedure affords relatively large metal crystallites concentrated at the surface of the support particle. For an example see M. Kotter et al. Delmon et al. editors Preparation of Catalysis II, Elsevier Scientific Publishing Company, New York (1979) p. 51-62.
As disclosed in the three references discussed below, a second version of the pore volume impregnation procedure involves a double impregnation technique. This procedure is similar to the single PVI procedure above, but in the first impregnation, a metal chelating agent is bound to the catalyst support. In the second impregnation, the pre-doped support is contacted with an aqueous metal salt solution, dried, and calcined as above. But this process, in general, does not yield catalysts characterized by high metal loading simultaneously with small metal crystallite size. Accordingly, high activities per unit metal are not achieved.
In the first reference, Barcicki et al. (React. Kinet. Catal. Lett., Vol 17, No. 1-2, 169-173 (1981))--incorporated herein by reference--discloses nickel catalysts characterized by small metal crystallites in the range of 20 to 30 .ANG. supported on .gamma.-alumina. But in the best case, the nickel loading was only about 3 weight percent.
In a second reference, Nazimek (Applied Catalysis 12 (1984) 227-236) presents a study of the alkane hydrogenolysis reaction rate dependence on nickel crystallite size. The nickel's average crystallite size ranges from about 15 .ANG. to about 120 .ANG., however, similar to the catalyst described above, the metal loading was only about 0.5 to about 2 weight percent.
In a third reference, WO 95/31280, another double impregnation procedure was disclosed.
The above single or double pore volume impregnation procedures allow manipulation of the aluminum oxide phase (e.g., calcination to achieve mechanically stable aluminum oxide) prior to metal introduction. But, if such a procedure is used to impregnate .gamma.-,.delta.-,.theta.-, and .alpha.-alumina, then high surface area, high metal loading, and small metal crystallite size are difficult to achieve. One explanation for this is that the more mechanically stable aluminum oxide phase supports have relatively low surface areas, and the art, in general, teaches that the lower the surface area of the aluminum oxide support, the more difficult it is to achieve small metal crystallite size, simultaneously with high loading of a subsequently impregnated metal.
Co-precipitation is another procedure for the preparation of aluminum oxide supported copper oxide crystallites. This procedure yields aluminum oxide supported catalysts with high surface area, high metal loading, and small metal crystallite size. Disadvantages are that co-precipitated type copper-aluminum oxide catalysts have low mechanical stability (i.e., the aluminum oxide support exists in the unstable amorphous phase rather than in the mechanically stable .gamma.-,.delta.-,.theta.-, or .alpha.-phases) and relatively low activity per unit copper metal. Even further, co-precipitated copper on aluminum oxide catalysts tend to leach copper during aqueous phase reactions. A high copper content in the effluent indicates a greater degree of leaching that, of course, shortens the catalyst's life and can have a negative impact on the quality of the final acrylamide product solution in commercial applications.
The co-precipitation procedure for making copper on aluminum oxide should be distinguished from the impregnation methods discussed above. With co-precipitation, the aluminum oxide-copper salt complex is formed directly in solution. Thus, pretreatment of the aluminum oxide phase (e.g., calcination to achieve mechanically stable .gamma.-,.delta.-,.theta.-, or .alpha.-aluminum oxide phases), before the metal is introduced, is impossible. So, in the co-precipitation case, while high surface area, high metal loading, and small metal crystallite size are obtainable, the aluminum oxide is formed in the unstable amorphous phase rather than in the mechanically stable .gamma.-,.delta.-,.theta.-, or .alpha.-phases. Any attempt to improve the stability of the co-precipitated type catalyst, by calcination at temperatures over about 650.degree. C., generally results in copper crystallite sintering. Concerning co-precipitated catalyst's low relative activity per unit copper metal, this may be due to partial occlusion of the copper source within the aluminum oxide matrix. Hence, the copper metal is not available for reaction. A more efficient arrangement is to have the active material distributed on the surface area throughout the particle. (Augustine R. L., Heterogeneous catalysis for the Synthetic Chemist, p. 271, Marcel Dekker, Inc. (1996)).
For an example see German laid open application DOS 2,445,303 (corresponding to U.S. Pat. Nos. 4,009,124 and 4,062,899), which discloses base treatment of an aqueous copper salt (e.g., copper nitrate) and aluminum salt (e.g., aluminum nitrate) solution to precipitate a mixture of copper and aluminum oxides. This precipitated powder is filtered and calcined at 350 to 600.degree. C. Additional patents describing co-precipitated catalysts include U.S. Pat. Nos. 5,817,872 4,631,266, and 4,386,018 and European Patent Application 0434,061 B1.
Another procedure, which can be utilized for the preparation of copper oxide crystallites on an aluminum oxide support, is called precipitation-deposition. The process comprises inducing precipitation of a dissolved metal species which then deposits upon a finely powdered solid support. Precipitation of the dissolved metal species can be induced by changing the solution pH, decomplexation to an insoluble metal ion, or oxidation or reduction techniques, etc.
The precipitation-deposition procedure is exemplified in U.S. Pat. No. 4,113,658 to Geus. Geus discloses a precipitation-deposition procedure to prepare supported metal catalysts characterized by small metal crystallite size (about 50 to 300 .ANG.) and high metal loading (e.g., 50% by weight or more). The final catalyst (i.e., metal species/support composite) is in the form of a fine powder of particle size between about 100 to about 3000 .ANG.. After the precipitation/deposition, the catalyst may be treated by subsequent heating, oxidation, or reduction to form other catalytically active species.
A primary disadvantage of precipitation/deposition is that finely powdered catalysts result. Catalysts in the form of fine powders are not suitable for fixed bed reactor use. Generally, for use in a fixed bed reactor, a catalyst pellet size of about 1.5 mm to about 10 mm in diameter is required. What is more, the fine powdered catalysts prepared by precipitation/deposition cannot be pressed and calcined into mechanically stable particles of suitable size for fixed bed reactor use and maintain small metal crystallite size and high activity on a per unit of metal basis. For a discussion of calcination relative to pellet strength and catalyst activity see Satterfield, Heterogeneous Catalysis in Practice, Chemical Engineering Series, p. 75 and pp. 136-141, McGraw-Hill, Inc., 1980.
Another disadvantage of catalysts prepared by precipitation-deposition is that the active metal is distributed primarily on the outer circumference (in a sort of "egg shell" arrangement) of the catalyst particles (see e.g., U.S. Pat. No. 4,113,658 column 2, lines 10-14 and Augustine R. L., Heterogeneous catalysis for the Synthetic Chemist, p. 278, Marcel Dekker, Inc. (1996)).
Supported copper oxide crystallites, for example, those prepared according to the above discussed methods, can be converted to supported elemental copper crystallites by chemical reduction. Most commonly the elemental copper crystallites are obtained by reduction of the copper oxide crystallites with hydrogen at elevated temperatures (150 to 250.degree. C. ). A major problem in the art is that during reduction, sintering of the copper metal species can lead to increased metal crystallite size and consequently the resulting supported elemental copper catalyst has low catalytic activity on a per unit copper basis. Thus even if small copper oxide crystallites at high copper loading on the support are achieved, reduction to the elemental copper form generally results in a supported elemental copper catalyst of relatively low activity per unit copper metal.
In summary, the prior art has failed to disclose a supported copper catalyst characterized by small metal crystallite size, high active surface area, high mechanical stability (i.e., the aluminum oxide support is present in the .gamma.-,.delta.-, .theta.-, or .alpha.-alumina phase), extended lifetime, high metal loading, and that is of particle size suitable for use in fixed bed type reactors. The prior art has also failed to disclose an aluminum oxide supported copper oxide catalyst, whereupon reduction affords a highly active aluminum oxide supported elemental copper catalyst characterized by high activity per unit metal. Such a catalyst would be especially useful for commercial hydration of nitrites to amides. These objectives are now fully met by the catalysts of the present invention as described below.